1、Lessons Learned Entry: 0981Lesson Info:a71 Lesson Number: 0981a71 Lesson Date: 2000-06-12a71 Submitting Organization: GSFCa71 Submitted by: Mike Sampson and Jay BrusseSubject: Reverse Polarity Concerns With Tantalum Capacitors Description of Driving Event: Most tantalum capacitors are sensitive to t
2、he polarity of the applied voltage. Incorrect installation and/or improper circuit application that subjects the capacitors to reverse bias may lead to performance degradation or catastrophic failure (short circuit) of the capacitor. Dry slug tantalum capacitors can survive being installed backwards
3、 for long periods of time (thousands of hours) when derated to 15% or less of rated voltage.Lesson(s) Learned: By nature of their construction, tantalum capacitors (both wet slug and dry slug styles) are polar devices that are susceptible to performance degradation and/or catastrophic failure if rev
4、erse biased. This behavioral characteristic has always been known, but recent experiences have reminded us of the importance of proper installation and circuit design when using this type of component. Historically, reverse polarity failures have been observed that resulted from:a71 Incorrect initia
5、l installation (part installed backwards)a71 Incorrect installation during rework (original part installed correctly, but replacement part put in backwards)a71 Correct circuit design, but incorrect artwork on the printed wiring board (part installed correctly according to the artwork, but actually i
6、n backwards)a71 Incorrect circuit design (printed wiring board artwork correct per circuit layout, but design error resulted in parts in backwards)The NASA EEE Parts Assurance Group (NEPAG) is currently investigating the reverse polarity behavior of tantalum dry slug chip capacitors because there is
7、 little definitive information in the literature on this subject. Preliminary results will be available in the last quarter of 2001.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Guidelines Regarding Reverse Bias Application of Tantalum CapacitorsWe
8、t Slug (Non-Solid) Tantalum Capacitors Ex.- MIL-PRF-39006 CLR79, CLR81, CLR90 and CLR91 typesa71 Capable of handling up to 3 volts of reverse polarity regardless of capacitor voltage rating. Leakage current characteristics will be higher than normal.a71 Exceeding 3 volts reverse bias will cause rapi
9、d deterioration of the capacitor leading to catastrophic failure (short circuit)a71 Polarity identification is generally indicated by a “+“ or a stripe on the anode termination. Also, hermetically sealed, tubular case, wet slug styles have a crimp towards one end of the case that may be used to iden
10、tify the anode (“+“) end. Exceptions or variations to these conventions may exist.Dry Slug (Solid) Tantalum Capacitors Ex. MIL-PRF-39003 and MIL-PRF-55365 CSR, CSS and CWR typesa71 Sensitive to reverse polarity but tend to be much less so than their wet slug counterparts.a71 May fail explosively und
11、er reverse bias, even catch fire.a71 May survive “indefinitely“ at room temperature with up to 15% of their voltage rating applied in reverse. Capacitors will have a higher leakage current than in the forward direction, but this may be undetectable in the majority of heavily derated circuit applicat
12、ions. The circuit may operate properly for many hours (hundreds or even thousands) before failure. Influential factors in life expectancy are believed to include: manufacturer, style, size, capacitance, moisture content and of course rated voltage.a71 “Chip“ styles may be more vulnerable to failure
13、under reverse bias than hermetically sealed styles because moisture may accelerate failure. NEPAG is investigating the effect of moisture.a71 “Chip“ styles are more prone to being installed improperly because they may be very small and the polarity identifier can be hard to see. a72 Conformally coat
14、ed “chip“ styles (ex.-CWR06) typically identify the polarity by a small projection “nub“ in the center of one termination representing the anode (+) terminationa72 Molded case “chip“ styles (ex.-CWR09 and CWR11) identify the polarity by a beveled edge in the case and/or a polarity bar marked on the
15、casea71 Polarity identification is generally indicated by a “+“ or a stripe adjacent to the anode termination. Exceptions or variations to these conventions may exist. One example of such an exception is the tubular case capacitor made in accordance with MIL-PRF-39003/10 which requires a stripe to i
16、dentify the cathode “-“ terminationProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Recommendation(s): Based on the aforementioned concerns with reverse bias application of tantalum capacitors, the following suggestions are provided.a71 Due to possibl
17、e variations in the conventions used to identify capacitor polarity, careful review of the capacitors marking requirements (per MIL specification or per manufacturer data sheet) must be performed to ensure proper orientation during installationa71 Carefully review circuit designs and printed wiring
18、board artwork for correct polarity of tantalum capacitorsa71 Carefully inspect all tantalum capacitors following installation to ensure their polarity matches the artwork (or corrective action instructions if the artwork is faulty)a71 Photograph populated boards at a magnification, fidelity and from
19、 a perspective capable of showing the polarity of the tantalum capacitorsa71 Carefully review the polarity of all tantalum chip capacitors replaced as a result of reworkEvidence of Recurrence Control Effectiveness: N/ADocuments Related to Lesson: N/AMission Directorate(s): a71 Exploration Systemsa71
20、 Sciencea71 Space Operationsa71 Aeronautics ResearchAdditional Key Phrase(s): a71 Aircrafta71 Communication Systemsa71 Computersa71 Flight Equipmenta71 Ground Equipmenta71 Hardwarea71 Packaging Handling Storagea71 Parts Materials & ProcessesProvided by IHSNot for ResaleNo reproduction or networking
21、permitted without license from IHS-,-,-a71 Risk Management/Assessmenta71 Safety & Mission AssuranceAdditional Info: Approval Info: a71 Approval Date: 2001-08-07a71 Approval Name: Jay Liebowitza71 Approval Organization: GSFCa71 Approval Phone Number: 301-286-4467Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
